The Fischer-Tropsch process can be used for the conversion of hydrocarbonaceous feedstocks into liquid and/or solid hydrocarbons. The feedstock (e.g. natural gas, associated gas, coal-bed methane, biomass, heavy oil residues, coal) is converted in a first step into a mixture of hydrogen and carbon monoxide (this mixture is often referred to as synthesis gas or syngas). The synthesis gas is then fed into a reactor where it is converted over a suitable catalyst at elevated temperature and pressure into paraffinic compounds ranging from methane to high molecular weight molecules comprising up to 200 carbon atoms, or, under particular circumstances, even more. Examples of the Fischer-Tropsch process are described in e.g. WO-A-02/02489, WO-A-01/76736, WO-A-02/07882, EP-A-510771 and EP-A-450861.
Numerous types of reactor systems have been developed for carrying out the Fischer-Tropsch reaction. For example, Fischer-Tropsch reactor systems include fixed bed reactors, especially multi-tubular fixed bed reactors, fluidised bed reactors, such as entrained fluidised bed reactors and fixed fluidised bed reactors, and slurry bed reactors such as three-phase slurry bubble columns and ebulated bed reactors.
As mentioned above, “coal” and heavy oil residues are examples of feedstocks for the Fischer-Tropsch process. However, there are many solid fossil fuels which may be used as feedstock for the process, including solid fuels such as anthracite, brown coal, bitumous coal, sub-bitumous coal, lignite, petroleum coke, peat and the like. All such types of fuels have different levels of ‘quality’, that is the proportions of hydrogen and carbon and, as well as substances regarded as ‘impurities’, generally sulfur and sulfur-based compounds, nitrogen containing compounds, ash, heavy metals etc.
Gasification of solid carbonaceous fuels such as coal is well known, and generally involves milling or otherwise grinding the fuel to a preferred size or size range, followed by reacting the fuel with oxygen in a gasifier. This creates the mixture of hydrogen and carbon monoxide referred to as syngas or synthesis gas. In many known processes, N2 is used as a transport gas for transporting the coal to the burner of the gasification reactor. A problem of the use of N2 as a transport gas is that the N2, although relatively inert, may lead to undesirably reducing the efficiency of the downstream catalysts. Moreover the presence of nitrogen will require more reactor volume for performing the Fischer-Tropsch synthesis at the same production capacity, especially when a synthesis gas recycle over the Fischer-Tropsch reactor is used.
U.S. Pat. No. 3,976,442 describes a process wherein a solid carbonaceous fuel is transported in a CO2 rich gas to a burner of a pressurized gasification reactor operating at about 50 bar. According to the examples of this publication a flow of coal and carbon dioxide at a weight ratio of CO2 to coal of about 1.0 is supplied to the annular passage of the annular burner at a velocity of 150 ft/sec. Oxygen is passed through the centre passage of the burner at a temperature of 300° F. and a velocity of 250 ft/sec. U.S. Pat. No. 3,976,442 thus provides a process wherein the partial oxidation is performed in a pressurized reactor and wherein the use of nitrogen as transport gas is avoided. Nevertheless the use of carbon dioxide as transport gas was never practiced or seriously considered in the intermediate 30 years. This was probably due to the low carbon efficiency of the process as disclosed by this publication. The low efficiency of the coal to the synthesis gas will ultimately effect the efficiency of the total process starting from coal to the products as obtained from the Fischer-Tropsch process.